If you are shitfaced, you have drunk so much alcohol that you can barely stand, talk, or think. It is informal American English, and it is crude. Friends might say it about a wild night; you would not say it in a classroom, a hospital, or to someone's parents. Though actually, it does not mean there is shit on your face. The faced piece is only a rough way of saying you can see it on the person — they are that far gone. It is about being extremely drunk, nothing else.

喝掛了、爛醉（粗話）